In order to improve system capacity, peak data rate and coverage reliability, the signal transmitted to and by a particular user is modified to account for the signal quality variation through a process commonly referred to as link adaptation. Adaptation Modulation and Coding (AMC) is a technology which is commonly used in communication system for achieving spectral efficient transmission. With AMC a communication system tries to select a modulation and coding scheme (MCS) which matches an instantaneous channel used by a cell. If a too aggressive MCS is used there is a severe risk that data transmitted via the channel will not be detected by the receiver while in case a too robust MCS is used, resources will likely be wasted on unnecessary redundancy.
AMC is often combined with a retransmission protocol to ensure a small grade of packet loss. In many communication systems, including Long Term Evolution (LTE) and High Speed Packet Access (HSPA), A Hybrid Automatic Repeat Request (HARQ) is used. With HARQ the receiver first tries to decode received data. If the receiver fails to decode the data it notifies the transmitter from which the data was sent, which will then resend the data. The receiver is then adapted to combine data received from both transmission attempts to try to perform a new decoding.
In order to maximize the spectral efficiency in a cell it is often promising to aim at having few re-transmissions. A retransmission rate of about 10% has proven to be good. A high number of retransmissions will increase the delay, since retransmissions are time consuming. However, transmissions performed with a too robust MCS may increase the delay in a typical multi-user system, since all or most data channels may be occupied.
In the 3GPP LTE Rel.8 standard both AMC and HARQ are suggested to be used for the purpose of obtaining an efficient link adaptation in both uplink (UL) and downlink (DL). According to this standard a User Equipment (UE) is also signaled a resource and MCS on the Physical Downlink Control Channel (PDCCH) both for UL transmission on the Physical Uplink Shared Channel (PUSCH) and DL Reception on the Physical Downlink Shared Channel (PDSCH). Retransmissions in the DL are also signaled on the PDCCH while retransmissions for the UL are signaled either on the PDCCH or triggered automatically by feeding back a negative HARQ acknowledgement (NACK) to the UE ON THE Physical HARG Indicator Channel (PHICH). If signaled on the PHICH the retransmission will occur with the parameters given for the previous transmission attempt.
Link adaptation may be optimized for spectral efficiency or for minimized delay or more commonly as a tradeoff between the two. It is also commonly known that different link adaptation mechanisms may be used for different types of traffic, where, by way of example, delay sensitive traffic, such as e.g. Voice over IP (VoIP), may have a lower Block Error Rate (BLER) than traffic associated with file download. It is commonly known that delay is not only affected by the link adaptation, but that also the scheduling of resources to users has an effect on the delay. If there are many users in a system the duration of time until scheduled for a user having data to send may have more effect on the delay than the transmission attempts required for the data delivery. A consequence from this fact is that a link adaptation obtained for and suitable for one traffic scenario will probably be very unsuitable for another traffic scenario.